[MailRaider](https://www.45rpmsoftware.com/software/mailraider.php) by Pascal Harris seems to work well with my initial use and, best of all, it's free:

> Are you a Switcher? Have you got old e-mails archived somewhere? Have you ever
> wished that your Mac could read your old Microsoft™ Outlook .msg files? If so,
> MailRaider could be just what you need. Try it. And let me know how you get on.

I've tested it with simple `.msg` files and ones with attachments and it seems to work very well.